The performance of the simple gas turbine engine cycle, whether measured in terms of efficiency or specific output, is improved by increasing the turbine gas temperature. It is therefore desirable to operate the turbine at the highest possible temperature. For any engine cycle compression ratio or bypass ratio, increasing the turbine entry gas temperature always produces more specific thrust (e.g. engine thrust per unit of air mass flow). However, as turbine entry temperatures increase, the life of an uncooled turbine falls, necessitating the development of better materials and the introduction of internal air cooling.
In modern engines, the high pressure (HP) turbine gas temperatures are now much hotter than the melting point of the blade materials used, and in some engine designs the intermediate pressure (IP) and low pressure (LP) turbines are also cooled. During its passage through the turbine, the mean temperature of the gas stream decreases as power is extracted. Therefore the need to cool the static and rotary parts of the engine structure decreases as the gas moves from the HP stage(s) through the IP and LP stages towards the exit nozzle.
Internal convection and external films are the main methods of cooling the aerofoils. HP turbine nozzle guide vanes (NGV's) consume the greatest amount of cooling air on high temperature engines. HP blades typically use about half of the NGV cooling air flow. The IP and LP stages downstream of the HP turbine use progressively less cooling air.
FIG. 1 shows an isometric view of a conventional HP stage cooled turbine. Block arrows indicate cooling air flows. The stage has NGVs 100 with inner 102 and outer 104 platforms and HP rotor blades 106 downstream of the NGVs. Upstream of the NGVs, a rear inner discharge nozzle (RIDN) 108 and a rear outer discharge nozzle (RODN) 110 are formed by respective sealing rings which bridge the gaps between end-walls (not shown) of the engine combustor and the platforms 102, 104. The RIDN and the RODN take up the relative axial and radial movement between the combustor and the NGVs.
The NGVs 100 and HP blades 106 are cooled by using high pressure air from the compressor that has by-passed the combustor and is therefore relatively cool compared to the working gas temperature. Typical cooling air temperatures are between 800 and 1000 K. Mainstream gas temperatures can be in excess of 2100 K.
The cooling air from the compressor that is used to cool the hot turbine components is not used fully to extract work from the turbine. Extracting coolant flow therefore has an adverse effect on the engine operating efficiency. It is thus important to use this cooling air as effectively as possible.
The radial gas temperature distribution supplied to the turbine from the combustor is relatively uniform from root to tip. This flat profile causes overheating problems to end-walls such as the NGV platforms 102, 104 and the blade platform 112 and shroud 114, which are difficult to cool due to the strong secondary flow fields that exist in these regions. In particular, such overheating can lead to premature spallation of thermal barrier coatings followed by oxidation of parent metal, and thermal fatigue cracking.
Any dedicated cooling flow used to cool the platforms and shroud, when reintroduced into the mainstream gas-path causes mixing losses which have a detrimental effect on the turbine stage efficiency. Thus an alternative approach is to modify the temperature profile over a radial traverse of the mainstream gas annulus by locally introducing relatively large quantities of dilution cooling air at a plane upstream of the NGV aerofoil leading edges, for example at the RIDN 108 and the RODN 110. This ballistic cooling flow penetrates the hot gas stream, due to the high angle at which the coolant is introduced, and mixes vigorously with the gas flow to locally reduce the gas temperature. The resulting peaky radial temperature profile heats up the aerofoil and cools down the end-walls, while maintaining the same average gas temperature into the NGVs.
Conventionally the ballistic flow introduced at the RIDN and RODN enters the mainstream gas-path relatively far upstream of the NGV aerofoil through circumferential rows of holes 116, arranged in a staggered formation in the respective sealing ring. The holes are drilled with a radial orientation such that the cooling air enters the mainstream gas-path in the same radial direction.
With engine cycle gas temperatures rising and combustion temperature profiles becoming flatter, as a consequence of the drive to reduce NOx and CO2 emissions, there is an increasing need to make better use of this cooling air.